The allegations underlying the case are pretty graphic. Kim Lee Millbrook, a federal prisoner who's serving a 31-year sentence, claimed that he was assaulted by prison guards in March 2010. Allegedly one guard forced him to perform oral sex on him, while another guard held Millbrook in a choke hold, and a third officer stood watch nearby.

Whether the claims are truthful is not at issue in this case. What was at issue was whether Millbrook could proceed with a lawsuit against the federal government under the Federal Tort Claims Act (FTCA).

Let's first talk about how the FTCA works. Under the law, the government allows itself to be sued when a government representative commits a tort. A tort is an act done negligently or intentionally that results in injury to someone. However, if the tort was intentional, the law does not allow the lawsuit to proceed — except in cases where the defendant is a law enforcement official. And even in those cases, the federal government can be liable only if the officer was acting "within the scope of his office or employment."

What does "scope of employment" mean? Some lower courts have confined the phrase to mean only when the officer was specifically engaged in investigative or law enforcement activity, or when the officer was making an arrest, seizing evidence or conducting a search. Millbrook argued that those lower court rulings are too narrow and wouldn't capture egregiously abusive conduct by prison officers in many instances.

The Court agreed the lower court rulings were too narrow, and held that people can sue the federal government under the FTCA for "acts or omissions of law enforcement officers that arise within the scope of their employment, regardless of whether the officers are engaged in investigative or law enforcement activity, or are executing a search, seizing evidence, or making an arrest."

Whether Millbrook, a frequent litigant against the federal government who handwrote his own petition for writ of certiorari, will actually win his case in the lower court is another matter left for a future day. The Court didn't touch the issue of whether sexual assault could conceivably fall within the "scope" of a prison guard's employment.

Chris Paolella, Millbrook's lawyer, said it could, and that's why today's ruling is so important.

"Reprehensible activity can be within the scope of employment – it's a reprehensible way of doing a job, but they're doing it to exercise authority over the people that are in their charge," said Paolella.

And he said being able to sue the federal government when prison guards commit terrible acts is an important way to deter that conduct.

"Basically the government now has a direct pocketbook interest in stopping this kind of behavior," he said.

But Jeff Bucholtz, the lawyer for the other side, said any deterrence benefits of today's decision are minimal at best.

"FTCA judgments and settlements are not paid by the person who engaged in the misconduct and they're not paid by that person's employing agency," Bucholtz pointed out.

Instead, Bucholtz explained, FTCA judgments are paid by an unlimited fund provided by Congress, so it doesn't hurt prison guards or their supervisors when judgments are paid out under the statute.